Electronic devices often incorporate controllable attenuation devices and/or components for varying the amount of resistance applied to electronic signals such as radio frequency (RF) signals. Such attenuators are used in, among other things, automatic gain control circuits, position locating systems, telephone systems, television systems, and microwave circuit applications.
Some controllable attenuation devices such as monolithic microwave integrated circuit (MMIC) digital attenuators incorporate high frequency field effect transistors (FETs), such as gallium arsenide (GaAs) metal semiconductor FETs, arranged in a variety of network configurations (which may include other circuit elements, e.g., discrete resistor, among others). These devices operate by turning certain transistors on and off to adjust or select the desired attenuation.
Digital attenuators vary the strength of input signals in response to digital control signals. In a typical 1-bit digital attenuator, the amount of attenuation offered by the attenuator varies depending on whether the bit of the control signal has a value of “0” (logic low) or “1” (logic high). Typically, if a 2-bit or other multiple-bit digital attenuator is desired, a plurality of 1-bit digital attenuators are cascaded according to known techniques to produce the desired m-bit digital attenuator (where m≧2). For example, if a 3-bit digital attenuator is desired, three of the 1-bit digital attenuators are cascaded to produce the 3-bit digital attenuator.
Due to the cascading of bits, however, a reference insertion loss in conventional multi-bit digital attenuators tends to be high resulting in higher Voltage Standing Wave Ratio (VSWR), which represents the amount of reflected power. A high VSWR increases noise, which degrades system performance. Further, having multiple bits that are cascaded in the digital attenuator deteriorates the attenuation accuracy when multiple bits are switched on at the same time.
For certain applications, it is desirable to have a constant phase over attenuation states. Usually, for lower bits and at lower frequencies, phase shift may be less and manageable using techniques such as cascading lower bits. At higher frequencies, however, conventional digital attenuators may experience significant phase difference between the on and off states.
Accordingly, there exists the need for an improved digital attenuator to provide constant phase at higher frequencies.